1. Field of the Invention
The invention generally relates to shutter mechanisms in photographic cameras useable with chemical flash lamps. More particularly, the invention relates to an improved shutter mechanism for synchronizing film exposure with the peak intensity of an ignited flash lamp.
2. Background Art-Problems
Relatively fast shutter speeds may be used in combination with modern high-speed amateur color film (e.g. ASA 400) to reduce the effects of camera and/or subject motion, thereby resulting in sharper pictures. Some popularly priced amateur cameras take advantage of the extra sharpness that can be achieved with high-speed films by having fast (e.g. 1/200 to 1/500 second) shutter speeds for daylight operation. Unfortunately, the performance of low priced amateur cameras employing chemical flash illumination for indoor operation typically has not similarly benefited from the advent of high-speed films. One popular format for chemical flash employs a flash firing mechanism including a piezoelectric crystal that is struck by a hammer to generate a pulse of electricity which ignites the contents of a chemical flash lamp. Generally, the piezoelectric crystal is struck at the same time that the shutter begins to open, or shortly before, as for example in published U.K. Pat. Appl. No. GB 2000307 and U.S. Pat. No. 4,183,648. FIG. 1 of the drawings shows a plot of the light intensity produced by a piezoelectrically ignited flash lamp versus time. At time 0, the piezoelectric crystal is struck. As can be seen in FIG. 1, it took approximately 13 milliseconds from the time that the piezoelectric crystal was struck to the instant when the output of the flash lamp reached its peak intensity. If a relatively fast shutter speed is employed (e.g. 2 to 10 milliseconds; 1/500 to 1/100 second) and the shutter begins to open upon the hammer striking the piezoelectric crystal, the shutter will close before the flash intensity has passed its peak. Much of the flashlight will therefore be wasted. To overcome this problem, many inexpensive cameras are designed to employ a relatively slow shutter speed (e.g. 30 milliseconds) in the flash mode. By using such a slow shutter speed, the shutter is sure to be open when the flash intensity passes its peak. Although this technique makes full use of the flash illumination available, it sacrifices some sharpness potential in the high speed film.
An improvement to using a slow shutter speed in the flash mode is to delay opening of the shutter until shortly before the piezoelectrically ignited flash lamp reaches its peak intensity, then to open the shutter for a relatively short exposure time in synchronization with the peak flash intensity. It has been a problem, however, to design a simple shutter mechanism that could be actuated about the same time as the flash lamp is ignited and, after an initial delay, would produce the desired relatively short exposure time. One such shutter mechanism is shown in U.S. Pat. No. 4,349,259, and includes a primary shutter blade having an intermediate opening and a flap-like auxiliary blade fixed on the primary blade adjacent the opening. In operation, the primary blade is actuated in synchronization with piezoelectric ignition of a flash lamp. A stationary cam folds the auxiliary blade over the opening in the primary blade as the opening passes over an exposure aperture, thereby delaying film exposure with respect to flash ignition. Once clear of the cam, the auxiliary blade straightens by virtue of its own restoring force to uncover the opening. Then, on spring-urged return movement of the primary blade, the opening passes over the exposure aperture to effect film exposure substantially simultaneously with the ignited flash reaching its peak intensity.
While the shutter mechanism in the '259 patent delays shutter opening after flash ignition to synchronize film exposure with the peak intensity of an ignited flash, it suffers from a number of problems. For example, in low ambient temperatures (e.g. 20.degree. F.) the auxiliary blade will become less flexible and lose some degree of its restoring ability, possibly to the point where it may contact the cam during return movement of the primary blade, thereby stalling the primary blade. Moreover the use of a single blade having an intermediate opening, i.e., the primary blade, for uncovering and recovering the exposure aperture, makes it difficult to accurately vary the exposure time.